The invention relates to an apparatus for forming magnetic resonance images, which apparatus includes
a gradient coil system that includes a gradient coil carrier on which gradient coils are arranged, said gradient coil carrier being attached to a frame of the apparatus by way of suspension elements, each of which is provided with a resilient element.
An apparatus of this kind is known from U.S. Pat. No. 5,793,210. The MRI apparatus that is described in the cited document is provided with a gradient coil system with gradient coils that are arranged in an enclosure in which the gas pressure is lower than that of the ambient atmosphere. Coils of this kind may be arranged on a gradient coil carrier which itself is attached to further parts of the MRI apparatus; the gradient coil carrier is attached notably to the frame of the MRI apparatus by way of suspension elements.
It is a generally known fact that gradient coils in operation produce noise that is very annoying to the patients to be examined. Therefore, the technical aim is to reduce this noise as much as possible. To this end, the gradient coils in the known MRI apparatus are arranged in a vacuum atmosphere with a residual pressure such that the acoustic transfer of vibrations, arising in the gradient, to the surroundings via said residual atmosphere is strongly reduced. Said vacuum space may be filled with a noise absorbing fiber glass material so as to reduce the residual transfer via the atmosphere even further. In order to counteract the transfer of vibrations via the suspension elements, the suspension elements are arranged so as to mitigate the transfer of vibrations that are generated by the gradient coil system. The gradient coil system is supported notably by materials that form part of the suspension elements and have the desired acoustic properties, for example rubber, plastic or an epoxy material, or by resilient elements that, like said materials, bear on rigid structural elements such as supports or flanges that are especially provided for this purpose.
It is known per se that the transfer of vibrations from a vibrating element to its surroundings can be counteracted by supporting said element by way of springs that have a comparatively low stiffness. The lower the stiffness of such a spring, the less the transfer of vibrations from the vibrating element (such as the gradient coil carrier) to the surroundings (such as the frame of the MRI apparatus) will be.
Simply supporting the gradient coil carrier by way of resilient elements, however, has some drawbacks. In particular there are still frequency ranges in which the vibrations of the gradient coil carrier still are such that they have disturbing effects.
It is an object of the invention to provide an MRI apparatus of the kind set forth in which the adverse effects of vibrations of the gradient coil carrier are counteracted better. To achieve this, the MRI apparatus in accordance with the invention is characterized in that each of the suspension elements is provided with an active drivable element that is connected in series with the resilient element. The invention is based on the recognition of the fact that an adequate reduction of the adverse effects of vibrations of the gradient coil carrier cannot be achieved merely by means of passive elements (that is, elements whose mechanical behavior responds exclusively to the static or dynamic load exerted thereon). When an active drivable element is inserted in the suspension elements, the drivable element in the suspension of the gradient coil carrier can be driven, via an electrical control circuit, in conformity with the special requirements that may be imposed on the design of an MRI apparatus in respect of vibration behavior.
A very weak suspension of the gradient coil carrier in conformity with the present state of the art notably has the drawback that in operation such a suspension may cause a change of position of the gradient coil carrier in a macroscopic sense; in other words, it may be that the gradient coil carrier is displaced in the same way as a rigid member. In dependence on the method of suspension of the gradient coil carrier, this displacement may become manifest, for example, as a macroscopic rotation of the gradient coil carrier or as a swinging movement on the suspension elements on which the gradient coil carrier bears. This drawback occurs notably in the case of a design that aims to realize an as weak as possible suspension with a view to achieving maximum vibration isolation. A macroscopic change of position means that during the acquisition of the MRI image the gradient fields are shifted relative to the object to be imaged; evidently, such a shift will have an adverse effect on the image quality. In order to counteract macroscopic displacements of the gradient coil carrier, an embodiment of the MRI apparatus in accordance with the invention is provided with a drive circuit that is arranged to drive the active drivable elements in such a manner that these elements neutralize the net forces acting on the gradient coil carrier. This aspect of the invention is based on the following insight. Vibrations of the gradient coil carrier are caused by the gradient currents in the steady magnetic field (the so-called main field) of the MRI apparatus. When the main field has the same strength and the same direction throughout the vicinity of the gradient coil carrier, the gradient coil carrier will not be subject to a net Lorentz force. However, because the main field is never homogeneous in practical circumstances, there will always be a net Lorentz force that causes the macroscopic vibrations and associated displacements. Such vibrations have a frequency that is typically of the order of magnitude of 10 Hz. When the active drivable elements are driven in such a manner that they deliver forces that are equal to and oppose said net forces, on balance no net forces will act on the gradient coil carrier any longer, so that macroscopic displacements can no longer occur either. Force compensation is thus applied in order to achieve this effect of elimination of the macroscopic displacements.
A further embodiment of the MRI apparatus in accordance with the invention is provided with a drive circuit that is arranged to drive the active drivable elements in such a manner that each of these elements compensates the vibration displacements performed by the gradient coil carrier at the area of the point of attachment of the relevant suspension element. This aspect of the invention is based on the following insight. The gradient carrier has at least one internal resonance frequency. For these resonance frequencies (the most important of which has a frequency that is typically of the order of magnitude of 700 Hz) the weak suspension constitutes an inadequate vibration isolation from the environment. This means that the transfer of vibrations that are produced by the gradient coil carrier to the environment (the frame of the MRI apparatus) is insufficiently reduced, so that the annoying noise is transferred to the further environment via the frame. This transfer is counteracted in that the active drivable element can be driven in such a manner that the displacement of the point of attachment of the suspension element to the gradient coil carrier, caused by the relevant vibrations, is compensated by an oppositely directed extension/shortening of the active drivable element that is caused by a change of the length of the drivable element that is induced by the driving. As a result, the suspension element is virtually weakened for the drive frequency (frequencies), notably for the stated typical value of 700 Hz. Such virtual weakening reduces the transfer of the dynamic forces that would be exerted on the frame of the MRI apparatus by the gradient coil carrier. Thus, displacement compensation is applied in order to achieve the effect of elimination of the transfer of forces from the gradient coil to the frame.
In order to realize (electronic) control with the desired characteristics, that is, force neutralization for low frequencies (with the result that macroscopic displacements are eliminated) and/or displacement compensation for higher frequencies (with the result that the transfer of force is prevented), an embodiment of the MRI apparatus in accordance with the invention is provided with a gradient control circuit for producing the gradient signal, the drive circuit in said apparatus being provided with a feedforward circuit that is connected between the gradient control circuit and the active drivable element. This embodiment advantageously utilizes the a priori knowledge concerning the state of vibration of the gradient coil carrier. This knowledge is derived from (the control signal for) the gradient currents in such a manner that a drive signal is generated for the active drivable element such that the desired force neutralization for low frequencies and/or the desired displacement compensation for higher frequencies are achieved.
The drive circuit in a further embodiment of the apparatus in accordance with the invention includes a feedback circuit that is arranged between a vibration sensor, provided at the area of the relevant point of attachment of the gradient coil carrier, and the active drivable element. Even though in theory the undesirable vibration of a given frequency can be adjusted to zero by means of feedforward control, in practical circumstances the result may be inadequate, for example, because the transfer of the relevant components is not accurately known or because it has changed in the course of time. In such cases the undesirable vibration can be advantageously measured by means of a vibration sensor so as to try and adjust it to zero by means of a feedback circuit. In practical circumstances this approach may offer a result that is equivalent to or even better than that of feedforward control. In this context a vibration sensor is to be understood to mean a sensor that picks up a characteristic vibration quantity, for example, a force sensor or a displacement sensor.